Recent advances in the control of AC electrical machines have depended upon evolution of the principle of field orientation. This control concept views the AC currents and flux linkages in the machine as rotating vectors in a two dimensional space (i.e., in a plane). The production of torque may be viewed as the interaction of the stator current vector and the rotor flux linkage vector. The required value of electro-magnetic torque together with a desirable value of air gap flux may be obtained by proper alignment of the stator current vector with respect to the rotor flux linkage vector. Since this control scheme requires orientation of the stator current with respect to the rotor flux, a key requirement of the control scheme is the computation of the instantaneous location of the rotating flux linkage vector. While the flux linkage vector may be easily located for a synchronous machine by using a rotor position sensor, locating the rotating flux linkage vector is considerably more difficult in an induction machine since the position of the rotor is not directly related to the rotor flux position because of the rotor slip.
Present schemes for the control of induction machines use the so-called indirect field orientation method in which the rotor flux is located only indirectly by measuring the rotor speed and calculating the rotor slip frequency. This approach is inherently inaccurate since it relies on a precise knowledge of the rotor resistance and inductance, quantities which change with changes in temperature and the saturation level in the iron of the machine. The problems associated with the indirect approach could be avoided by direct measurement of the rotor flux or, alternatively, the air gap flux. Present state of the art, however, requires the use of either search coils or Hall sensors to measure rotor flux or air gap flux.
Search coils are small coils made of very thin wire which are placed in selected slots of the stator of the machine. The voltages induced in these coils are sensed and then integrated to produce a measure of the flux linking the coils. Incorporating such flux coils in a machine necessarily requires extra operations in the manufacture of the armature winding of the machine and therefore increases the cost of the machine. In addition, since the coils are formed of very thin wire (e.g., approximately 40 gauge), they are subject to breakage over time due to vibration or continual flexing. Repair of the search coils requires removal of the tooth top insulator strip from the stator slots containing the flux search coil. In addition to being expensive, such a repair procedure can contribute to insulation failure of the motor coils upon reassembly. Hall sensors are also unreliable as they are very sensitive to temperature. Because the armature coils are one of the hottest parts within the machine, placement of Hall sensors in the stator armature has rarely, if ever, been attempted on a commercial level. Thus, a need has existed for a low cost and reliable way of sensing flux in an induction machine as a key to solving many of the problems inherent in the present designs of variable frequency induction motor drives.